Activated sludge-ozone treatability of tar wastewater and tar wastewater containing herbicide alachlor was studied. The microbial yield coefficients of activated sludge process for the tar wastewater increased with decreasing feed wastewater strength. However, the yield coefficient decreased for the alachlor wastewater. For the ozone oxidation of activated sludge effluent, COD removal efficiencies were 41% and 36% for the tar and alachlor wastewater, respectively, at O₃/COD ratio of 3.